Mechanism of Translocation

Several competing hypotheses have tried to explain the actual physical mechanism by which phloem moves its dissolved solutes. The simple diffusion hypothesis, proposing translocation by ordinary concentration-gradient diffusion, was rejected because measured translocation speeds are far higher than diffusion alone could achieve, and because translocation is a genuinely biological process that any metabolic poison can halt - something plain diffusion would not be sensitive to. The activated diffusion theory (Mason and Maskell, 1936) proposed that diffusion inside the sieve tube is somehow sped up, either by activating the diffusing molecules themselves or by reducing the protoplasm's resistance to their movement. The electro-osmotic theory (Fenson, 1957; Spanner, 1958) proposed instead that an electric potential difference across the sieve plate itself drives the coordinated movement of water together with dissolved solutes, but this theory too failed to explain several observed features of real translocation. The currently accepted explanation is Munch's mass flow (or pressure-flow) hypothesis, first proposed in 1930 and elaborated by Crafts in 1938: organic solutes move in bulk from a region of high osmotic pressure (the source) toward a region of low osmotic pressure (the sink), driven purely along a turgor-pressure gradient. Munch demonstrated the principle physically with two membrane-bound chambers, A (concentrated sugar solution) and B (dilute sugar solution), connected by a tube T and both immersed in a water reservoir: chamber A draws in water by endosmosis and its turgor pressure rises, which forces a mass flow of sugar solution from A to B through the tube, continuing until both chambers reach an isotonic balance - refreshed only if new sugar solution is added back into A. In a real plant, mesophyll cells (analogous to chamber A, the source) draw water osmotically from the adjoining xylem and build up turgor pressure, driving bulk flow of solutes through the sieve tubes (analogous to tube T) toward the lower-turgor cells of stem and root (analogous to chamber B, the sink), where the solutes are consumed or stored and any excess water is released back into the xylem through the cambium. Supporting evidence includes the sugar-rich exudate seen oozing from a freshly girdled stem and the loss of the normal source-to-sink concentration gradient in a defol …